The depth of absorption of a photon in, for example silicon, is dependent on the wavelength of the photon. The shorter the wavelength, the greater the energy level, the more likely the photon is absorbed by silicon. As an example, a near ultraviolet (NUV) photon is typically absorbed within about the first 1 μm of silicon. Red and green photons with lower energy levels than NUV photons are more likely to be absorbed at a deeper depth, such as within about 2 μm and 3 μm of silicon.
A photodiode is a semiconductor device that typically has a p-type semiconductor region and an n-type semiconductor region. The boundary between the p-region and n-region forms the p-n junction of the photodiode. During operation, the photodiode is in reverse bias mode. When the reverse biased photodiode is exposed to photons, electron-hole pairs are generated around the p-n junction of the photodiode. The electron-hole pairs formed around the p-n junction are swept away to the respective anode and cathode of the photodiode and a photocurrent can be measured.
Due to the capacitance of the photodiode, the voltage signal generated at the photodiode, is inversely related to the capacitance of the p-n junction and directly related to the photocurrent. The capacitance of the junction is inversely related to the width of the depletion region in the p-n junction because mobile charges at the edges of the depletion region respond to an applied voltage. The width of the depletion region is dependent on the doping concentration of the p and n-regions as well as the applied voltage.
The collection efficiency of a photodiode is a measurement of the number of generated electron hole pairs to the number of incident photons and also the number of charge carriers collected at the measuring node with respect to the number of generated electron hole pairs. The quantum efficiency of a photodiode is a measurement of the collected electron hole pairs with respect to the number of incident photons.
In a silicon photodiode, designed for visible light, the light absorbing portion of the photodiode has a depth of about 2 μm and 3 μm. In such a structure, if light is absorbed in a shallow surface region (i.e., NUV), charge carriers that are generated from the absorbed light are more likely to re-combine before reaching the photo-collection region of the photodiode, therefore reducing its collection efficiency.